Temperature compensated microwave device



July 5, 1949. E. G. LINDER 2,475,035 n TEMPERATURE COMPENSATED MICROWAVE DEVICE Filed Nov.l 8, 1944 I/ L? 10am/247m 0546 I N VEN TOR.

BY @M Patented July 5, 1949 2,475,035 TEMPERATURE COMPENSATED MICROWAVE DEVICE l Ernest G. Linder, Princeton, N. J., asslgnor to Radio Corporation of America, a corporation of Delaware Application November 8, 1944, Serial No. 562,426

Claims.

This invention relates generally to microwave communication systems and more particularly to an improved method of and means for temperature compensating microwave devices.

It is well known that the reactance of various microwave reactive devices such, for example, as cavity resonators, tuned lines, and other resonant structures varies as a function of dimensional changes in the conductive members thereof due to temperature variations. Also, it is well known that certain gases, some of which are good dielectrics at microwave frequencies, have dielectric constants which decrease with increasing temperature.

The instant invention contemplates an improved method of and means for compensating for variations in the reactance of totally enclosed microwave reactive structures caused by variations in the dimensions thereof due to temperature changes, by introducing into said structures a gaseous dielectric in which the dielectric constant varies with temperature in opposite sense to the dimensional changes in said structures. The degree of reactance-temperature compensation provided thereby may be adjusted by controlling the pressure of the particular gas introduced as a function of the electric moment of the particular gas.

Among the objects of the invention are to provide an improved method of and means for cornpensating for variations in reactance due to temperature changes in microwave structures. Another object of the invention is to provide an improved method of and means for compensating for variations in reactance due to dimensional changes with temperature in enclosed microwave reactive devices by introducing into said devices predetermined gaseous dielectrics in which the dielectric constant varies inversely with temperature. A further object of the invention is to provide an improved method and means for compensating for variations in reactance with temperature in enclosed microwave reactive devices by introducing a gaseous dielectric into said devices wherein the dielectric constanct varies with temperature in opposite sense to reactance variations due to thermal expansion and by adjusting the pressure of the introduced gas to provide the desired degree of reactance-temperature compensation.

An additional object of the invention is to provide an improved method of and means for compensating for tuning variations in enclosed microwave resonant devices by introducing therein a gaseous dielectric which varies with temperature in opposite sense to said tuning, and by adjusting thepressurefofsaid introduced gas to provide the desired degree of tuning-temperature compensation as a function' of the electric moment of the particular gas introduced.

The invention will be described in further detail by reference 'to the accompanying drawing of which Figure 1 is a schematic View of a preferred embodiment thereof and Figure 2 is a fragmentary schematic View of a modication thereof. Similar reference characters are applied to similar elements throughout the draw- Referring to Figure 1, a typical rectangular cavity resonator I includes, for example, a coupling loop 3 extending through insulators 5, into the interior of the resonator. The loop may be connected to any desired utilization device, not shown.` The interior of the resonator I is filled with a gaseous dielectric of any of the types described hereinafter which have a dielectric constant which varies with temperature in a manner to compensate for reactance variations due to dimensional changes in the resonator with variations in temperature. Gas of the desired type from a gas source 'I may be introduced into the resonator through a port 9 terminating a conduit II which includes, for example, a valve I3 and a pressure gauge I5. As explained hereinafter, the gas pressure in the resonator I may be adjusted to a value to provide the desired degree of reactance or frequency compensation. The port 9 may be sealed foff from or connected permanently to the gas conduit II, as desired.

Figure 2 illustrates the manner in which the invention may be employed to compensate for reactive variations with temperature in a typical coaxial line. 'I'he line includes a center conductor I1 and a coaXially-disposed outer cylindrical conductor I9 which is separated from the inner conductor I1 by means of a plurality of insulating spacers 2|, 23. The end spacers 2I may provide a gas seal for the gas introduced into the line through the port 9 terminating the gas line II. The port 9 may be sealed off from or permanently connected to the gas line II. If desired, the interior `walls of the outer conductor I9 may be thermally insulated from the enclosedA gas by means of a thermal insulating layer 25. Thermal insulation also may be provided for the inner walls of the cavity resonator I of the device illustrated'in Fig. 1.

Frequency-temperature compensation of standard frequency microwave devices, such, for example, as cavity resonators, coaxial lines or wavemeters is highly desirable due to the appreciable dimensional changes in conductive microwave tuning elements with temperature. A typical example of apparatus in which temperature compensation is especially advantageous is that of air-borne equipment which is flown through wide altitude ranges in tropical regions.

Since it can be shownthat the resonant frequency of a cavity resonator varies as an inverse function of the square root of the dielectric coni stant of the dielectric medium enclosed therein, it is proposed to compensate/for the dimensional changes in the resonator structure by selecting a gaseous dielectric which has a dielectric constant which decreases with increasing temperature. The gas pressure required for satisfactory compensation will depend upon the particular gaseous dielectric selected, as will be shown in greater detail hereinafter.

For gases having a permanent dielectric moment , Ing? (1 e1=41rn cl0+m where e is the dielectric constant, iris the number of molecules per'cubic centimeter, at is the limiting polarizability as T 'approaches infinity, o is the permanent electric: moment of the gas, 7c is the Boltzmann constant, vand T is the absolute temperature.

This form of the Clausius-Mossotti law clearly indicates that the dielectric constant e of particular `gases decreases with temperature. In a typical cavity resonator having a frequency fo when evacuated, the frequency f; When filled with ar medium of dielectric constant e, will be Thus, if a cavity resonator is filled with such a gas, its resonant frequency will increase with temperature due to th^eiect of a lowered dielectric constant of the gaaand this change will compensate for the effect of dimensional changes which ordinarily lower the resonant frequency with increases in temperature.

The rate of frequency-change with temperature of the cavity resonatorfmay Ibe 'obtained by differentiating Formula 2, whereby @j jr is (3) dT" 2 e wdr from Equation 1: v

- f ngi E1+41rn a0+3kT and Hence -a 1 l gli 4) e l'-1 MGD-L3M) and the corresponding rate `oi? vchange 4 of the dielectric constant e with temperature is de 41r7g2 (5) W- 31m Therefore the cavity resonator due to dimensional changes in the conductive elements thereof is in which l is a linear dimension of the cavity resonator, is the coefficient of linear thermal expansion of the cavity resonator walls and lo is the corresponding linear dimension at a reference temperature To, Therefore,

(10) f=f0[1(T- Toi] and the rate of change of frequency due to thermal expansion may be expressed The magnitude of the required electric moment g to provide a gas having desired temperature compensation characteristics may be obtained from Formulas 7 and 11, whence For a typical cavity resonator comprising copper or silver plated walls of Invar steel, the coefficient of linear thermal expansion is equal approximately with lO-S.

Therefore,

Numerous gases have electric moments of the order of magnitude indicated by Formula 14. The following are typical:

Gas: Electric moment (y) Hydrogen chloride 1.03X10-1 Hydrogen sulfide 0.95 10-18 Sulphur dioxide 1.61 10-18 Carbonyl sulfide 0.65 1018 Carbon monoxide 0.11 X 1018 Since most microwave applications require that cavity resonators exhibit a high ratio of reactance-to-resistance (Q) at microwave frequencies, it is also necessary that the particularga's introduced as a dielectric shall introduce no appreciable microwave losses, that is, the gas must have low electric conductivity. Of the specific gases listed heretofore, carbonyl sulfide and carbon monoxide are typical of dielectrics which introduce relatively small losses in a cavity resonator or microwave reactive device. Since none of the gases available will be found to have the identical electric moment computed heretofore in Formula 14, the differences therein may be compensated by adjusting the gas pressure, since it is evident from Formula 7 that is directly proportional to n which, in turn is directly proportional to the gas pressure. Therefore, carbonyl sulfide, at a pressure of of an atmosphere, or carbon monoxide, at a pressure of atmospheres, would satisfy the typical conditions described heretofore. Obviously, other gases at suitable pressures may be employed in the same manner. If the particular gas employed happens to have a corrosive action upon the walls of the cavity resonator or line, the walls may be coated or separated, such as by plastic coatings or glass layers, from the corrosive gas.

It should be understood that the method of and means for compensating for frequency or reactive variations in microwave devices by the introduction of suitable gaseous dielectrics at predetermined pressures may be employed for compensating all such types of microwave tuned structures whether such structures are operated at resonance or at some other frequency.

Thus the invention described comprises an improved method of and means for effectively compensating for dimensional `changes in enclosed microwave devices by introducing therein predetermined gaseous dielectrics in which the dielectric constant varies with temperature in the opposite sense to frequency changes due to dimensional variations in the structure, and wherein the pressure of the introduced gas is adjusted to provide the desired degree of compensation.

I claim as my invention:

1. In an enclosed microwave reactive device, the method of compensating for variations of reactance in said device with variations in the dimensions thereof due to changes in temperature which comprise introducing a gaseous dielectric into said device, said dielectric having a dielectric constant which varies reactance with temperature in opposite sense to said reactance variations, and adjusting the pressure of said introduced gas to provide the desired degree of reactance-temperature compensation in said device.

2. In an enclosed microwave resonant device, the method of compensating for variations in the resonant frequency of said device with variations in the dimensions thereof due to temperature changes comprising introducing a gaseous dielectric into said device, said dielectric having a dielectric constant which varies frequency with temperature in opposite sense to said frequency variations, and adjusting the pressure of said introduced gas to provide a predetermined degree of frequency-temperature compensation in said device.

3. In an enclosed microwave resonant device, the method of compensating for variations of the resonant frequency of said device with variations in the dimensions thereof due to variations of temperature comprising introducing a gaseous dielectric into said device, said dielectric having a 6. dielectric constant e which varies with absolute temperature T in accordance with the relation where n is a function of the gas pressure, o is the limiting polarizability of the gas as T approaches infinity, g is the permanent electric moment and lc is the Boltzmann constant of the gas, and adjusting the pressure of said introduced gas to provide the desired degree of frequency-temperature compensation.

4. In an enclosed microwave resonant device, the method of compensating for variations of the resonant frequency of said device with variations in the dimensions thereof due to temperature variations which comprises introducing into said device carbon monoxide gas as a dielectric, and adjusting the pressure of said gas to provide the desired degree of frequency-temperature compensation in said device.

5. In an enclosed microwave resonant device, the method of compensating for variations of the resonant frequency of said device due to temperature variations comprising introducing into said device hydrogen sulde gas as a dielectric, and adjusting the pressure of said gas to provide the desired degree of frequency-temperature compensation in said device.

6. In an enclosed microwave resonant device, the method of compensating for variations of the resonant frequency of said device with variations in temperature comprising introducing into said device carbonyl sulfide gas as a dielectric, and adjusting the pressure of said gas to provide the desired degree of frequency-temperature compensation in said device.

7. Temperature-compensated electrical apparatus including an enclosed reactive device, and a gaseous dielectric in said device, said dielectric having a dielectric constant which varies with temperature in opposite sense and in suitable magnitude substantially to compensate for reactance variations due to dimensional changes in said device with temperature, and said gaseous dielectric being under predetermined pressure to provide the desired degree of reactance-temperature compensation in said device.

8. Temperature-compensated microwave resonant apparatus including an enclosed resonant device, and a gaseous dielectric in said device, said dielectric having a dielectric constant which varies with temperature in opposite sense and in magnitude to compensate for the resonant frequency variations of said device due to dimensional changes therein with temperature, and said gaseous dielectric being under predetermined pressure to provide the desired degree of frequency-temperature compensation in said device.

9. Temperature-compensated electrical apparatus including an enclosed coaxial line, and a gaseous dielectric in said line, said dielectric having a dielectric constant which varies with temperature in opposite sense and in magnitude to compensate for reactance variations due to dimensional changes in said line with temperature, and said gaseous dielectric being under predetermined pressure to provide the desired degree of reactance-temperature compensation in said line.

10. Temperature-compensated electrical apparatus including an enclosed cavity resonator, and a gaseous dielectric in said resonator, said dielectric having a dielectric constant which varies with temperature in opposite sense and in magnitude to compensate for reactance variations due to dimensional changesl in; saidresonatorwith temperature, and meansifor adjusting the-pressure of said gaseous dielectric to vary the degree of reactance-temperature compensation in said resonator.

11. A completely enclosed microwave reactive device includingconductive reactive elements enclosing a gaseous dielectric having a dielectric constant which varieswith temperature in opposite sense and in. suitabler magnitude to compensate for, reactance variations due to dimen.- sional changes in said elements with temperature, and said gaseous dielectric being under` predetermined pressure to yprovide the desired degree of reactance-.temperature compensation in said device.

12. Inanenclosed microwave resonant device, the methQdof compensating for variations of the resonant frequency of;V said device with variations in the dimensions thereof' due to temperature variations which comprises introducing into said device as a dielectric algas ofthe group including carbon monoxide, hydrogen chloride,v hydrogen sulde, carbonyl'sullide, and'sulphur dioxide, and adjusting the pressure of'said gas to provide the desireddegree of Ifrequencytemperai,ure compensation in said' device.

13. In an enclosed microwave resonant device, the method ofcompensating for variations of the resonantl frequency of said device with variations in the dimensions thereof' due to4 temperature variations which comprises introducing into said device as a dielectric;v a gas of the group having a relatively high dielectric constant and a dipole moment, andA adjusting the pressure of said gas to providethe desiredv degree ofy frequency-temperaturecompensation in said device. e

14. Al completely enclosed microwave reactive device including conductive-reactive'elements enclosing a gaseous dielectric of the group including carbon monoxide, hydrogen choride, hydrogen f sulphide, carbonyl sulphide and sulphur dioxide,

said dielectric having a dielectric constant which varies with temperature in opposite sense and in suitable magnitude to compensate for reactance variations due to dimensional changes in said elements with temperature, and said gaseous dielectric being under predetermined pressure to provide the desired degree of reactance-temperature compensation in said device.

15. A completely enclosed microwave reactive device including conductive reactive elements enclosing a dielectric comprising a gas of the group having a relatively high dielectric constant and a dipole moment, said gas having a dielectric constant which varies with temperature in opposite sense and in suitable magnitude to compensate for reactance variations due to dimensional changes in saidelements with temperature, and said gaseous dielectric being under predetermined pressure to provide the desired degree of reactance-temperature compensation in said device.

ERNEST G. LINDER.

REFERENCES CITED UNITED STATES PATENTS Name Date Conklin et al July 19, 1938 Number 

